As is well known, magnets have a pair of poles, generally designated a north pole and a south pole. Like poles repel each other while opposite poles are attracted. Magnets attract ferromagnetic objects such as iron, nickel, cobalt and gadolinium. Ferromagnetic objects are made up of small regions called domains. Each domain may behave in the same manner as a small magnet having two poles, a north pole and a south pole. In an unmagnetized piece of ferromagnetic material, the domains are randomly arranged, thus canceling the magnetic effect. When a magnet is placed in close proximity to an unmagnetized piece of ferromagnetic material, however, the domains become temporarily aligned. The temporary alignment of the domains causes the north pole of the domains to attract the south pole of the magnet, and visa versa.
It is a well known law of physics that an electrical current produces a magnetic field. In a straight segment of wire carrying an electrical current, the magnetic field forms a cylindrical region having the wire as its central axis. When a wire forms a circle or loop, the loop creates a magnetic field that circumscribes the wire loop. The ends of the magnetic field created by the looped wire carrying current resembles a magnet; the end where the magnetic field enters acts as a south pole and the end where the magnetic field exits acts as a north pole.
A long coil of wire consisting of multiple loops is referred to as a solenoid. The magnetic field strength of a solenoid is the sum of the fields created by each individual loop, multiplied by the amperes running through the wire. Placing a piece of iron in the center of a solenoid creates an electromagnet. The iron greatly increases the magnetic strength of the solenoid because the domains in the iron become aligned by the magnetic field created by the current. Thus, the resulting magnetic field is the sum of the current running through the circular wire plus the magnetic field created by the aligned domains in the iron. The iron typically used in electromagnets is referred to as soft iron because it quickly loses its magnetism once the current in the wire is cut off and quickly regains magnetism once the current is turned on.
Electromagnets of the type described above are commonly used to lift large ferromagnetic objects. In the steel industry, for example, large electromagnets are used to pick up large plates of hot steel as they come off the press. The temperature of the steel plates can exceed temperatures of 1100-1200.degree. F. These extreme temperatures create several problems for electromagnets.
First, ferromagnetic materials, such as steel, lose their magnetic characteristics at a temperature known as the Curie temperature or point (about 1400.degree. F). At the Curie point, the high temperature re-randomizes the domains in the ferromagnetic material. Thus, a greater magnetic force is required to lift hot steel than cold steel. Moreover, as heat from the steel plates is transferred to the iron core of the electromagnet, the electromagnet itself loses some of its magnetic power.
Second, the heat from the steel plates damages the insulation surrounding the electrical wire coiled around the iron core of the electromagnet. The increasing temperature of the electrical wire is aggravated by heat generated by the current running through the electrical wire itself. The heat (or power) generated by the electrical coil is defined by the equation I.sup.2 R, where I is amperage and R is resistance. Thus, an increase in either amperage or resistance increases the heat generated by the electrical coil. With time, the excessive heat damages the insulation around the electrical wire. Eventually, the wire short circuits and the electromagnet loses its magnetic properties.
It will be appreciated that to repair electromagnets whose electrical wire has been damaged by heat, the electromagnet must be disassembled in order to gain access to the iron core and coiled electrical wire. And, once the electrical wire is replaced, the electromagnet must be reassembled. This seemingly easy task is made exceedingly arduous and tedious by conventional electromagnets which are designed with upwards of 24 bolts. To save time, rather than unscrewing the bolts, technicians often cut the bolts with a torch and replace them with new ones. Reassembly is also tedious. Prior art electromagnet designs require that the bolt holes in multiple heavy cast iron plates be perfectly aligned with the bolt holes in the magnet housing before the electromagnet can be assembled. At times, the task of properly aligning all the of metal plates of the electromagnet takes multiple technicians several hours. During this time the production line is often paralyzed.
From the foregoing, it will be appreciated that it would be an advancement in the art to provide an electromagnet that is simple, easy to assemble and disassemble, and inexpensive to build and maintain. In that regard it would be an advancement in the art to provide an electromagnet that could be easily and quickly assembled and disassembled, reducing the amount of down time resulting from damaged electrical coils. To that end, it would be an advancement in the art if the electromagnet design contained fewer bolts. It would be a further advancement in the art if the bolt holes could be easily aligned without the use of heavy lifting equipment. Such an electromagnet design is disclosed and claimed herein.